1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to an active matrix type liquid crystal display device which adopts an In-plane switching system mode as a display mode having a wide viewing angle.
2. Description of the Related Art
The In-plane switching (hereinafter also called IPS) mode which is one of display modes of an active matrix type liquid crystal display device has extremely wide viewing angle characteristics compared with the twisted nematic (hereinafter also called TN) mode which is currently the main stream. The IPS mode can realize a contrast ratio of not less than 10 at a viewing angle of 80xc2x0 in upward and downward directions as well as in left and right directions without using a particular optical compensation film. Accordingly, it is expected that the IPS mode will be popularly used by display devices having wide screens such as OA monitors and televisions.
FIGS. 3(a) and 3(b) are schematic views showing the constitution and the operation principle of the conventional typical liquid crystal display device adopting the IPS mode, wherein FIG. 3(a) shows an OFF state and FIG. 3(b) shows an ON state. In the drawing, numeral 2 indicates comb-like electrodes (only central portions of a pair of comb-like electrodes are shown), numeral 3 indicates liquid crystal molecules, numeral 5 indicates an electrode substrate, numeral 6 indicates a counter substrate, numeral 7 indicates an alignment layer, numeral 8a indicates a polarization film disposed at the counter substrate side, numeral 8b indicates a polarization film disposed at the electrode substrate side, numeral 9a indicates a light transmission axis of the polarization film 8a, numeral 9b indicates a light transmission axis of the polarization film 8b, numeral 10 indicates a backlight, and numeral 11 indicates a direction of an applied electric field. Besides the above-mentioned constitution, between the alignment layer 7 and the electrode substrate 5 as well as between the alignment layer 7 and the comb-like electrodes 2, insulation films may be interposed. Further, the comb-like electrodes 2 may be formed on the counter substrate side. As the material of the comb-like electrodes, an opaque material such as chromium or aluminum is used. This is because, the liquid crystal molecules rise up on the electrode due to the electric field in the vertical direction and hence, when the transparent material is used, the viewing angle characteristics are deteriorated.
Here, an alignment layer treatment such as rubbing is applied to the surface of the alignment layer such that the alignment direction makes an angle of xcex81 relative to the long side direction of the comb-like electrodes and hence, the liquid crystal molecules in the OFF state where the voltage is not applied take a homogeneously aligned state and such an angle xcex81 is set to approximately 5xc2x0 in the drawing. Further, the liquid crystal is tilted at approximately 3xc2x0 in an up and down direction toward the counter substrate 6 side and this tilting is called a pretilt angle. Further, the polarization film 8a has its light transmission axis arranged in parallel with the alignment direction of the liquid crystal, while the polarization film 8b is arranged such that the film 8b and 8a become crossed nicols. That is, in FIG.3(a), the transmission axis 9a of the polarization film 8a becomes equal to the direction of the rubbing treatment.
In the ON state shown in FIG. 3(b), the twist deformation is induced in the liquid crystal layer due to the transverse electric field 11 applied perpendicular to the long side direction of the comb-like electrodes. Due to the birefringence effect of the liquid crystal layer caused by the twist deformation, the liquid crystal layer wholly or partially transmits the light and the transmittance thereof is controlled by the magnitude of the twisting of the liquid crystal.
The detail of such an operation principle is, for example, described in the paper xe2x80x9cLiquid Crystals 22(4), pp. 379-390(1997)xe2x80x9d written by M. Oh-e et al. Further, although the drawing shows the case that the Np type liquid crystal is used, there may be a case that the Nn type liquid crystal is used. Compared to the case which uses the Np type liquid crystal, in the case which uses the Nn type liquid crystal, the alignment layer treatment is performed in a 90-degrees rotated direction.
However, in the conventional active matrix type liquid crystal display device which adopts the IPS mode as the display mode, since the electric field is applied radially to the distal ends of the comb-like electrodes, there has been a problem that a reverse twisted region where the liquid crystal is twisted in the reverse direction is partially formed and hence, a disclination occurs in the periphery of the region.
FIG. 4 is a view which schematically shows the comb-like electrodes used in the liquid crystal display device which adopts the IPS mode. In the drawing, numeral 4 indicates the alignment treatment direction, numerals 11a, 11b, 11c indicate the direction of electric field, and numeral 13 indicates the disclination. In the central portions of the comb-like electrodes 2, as depicted by the direction 11c of electric field, the electric field is applied in the direction substantially perpendicular to the long side direction of the comb-like electrodes and hence, no reverse twist occurs. At the distal ends of the comb-like electrodes 2, however, as depicted by the directions 11a, 11b of electric field, the electric field is applied radially and hence the reverse twisting region 12 is formed by the electric field having the direction 11a of the electric field. Although the electric field having the direction 11b of the electric field is applied radially, as shown in the drawing, since the applying direction is aligned with the rubbing direction, such an application of electric field does not form the reverse twisting region.
Further, as shown in FIG. 4, in the conventional IPS system, the homogeneous alignment layer treatment is performed in the direction which makes xcex82 relative to the long sides of the comb-like electrodes to control the twisting direction of the liquid crystal layer. However, there has been a problem that in case such homogeneous alignment layer treatment is performed, the viewing angle characteristics such as transmittance, contrast and the like do not become symmetrical in up and down directions as well as in left and right directions.
FIG. 5 shows the viewing angle dependency of the contrast in case the direction of the alignment layer treatment is set to 10xc2x0, wherein numeral 20 indicates equi-contrast lines of the contrast ratio 10 and numeral 21 indicates equi-contrast lines of the contrast ratio 15. As can be understood from the drawing, are rotated by the angle xcex83 (=10xc2x0) and hence, the equi-contrast lines are asymmetrical in up and down directions as well as in left and right directions. In the conventional liquid crystal display device which uses the comb-like electrodes, the viewing angle characteristics are not less than 80xc2x0 in an up and down direction as well as in a left and right direction and the contrast ratio is not less than 10. However, if the equi-contrast lines are symmetric in an up and down direction as well as in a left and right direction, substantial viewing angle characteristics as the display device are further enhanced.
In addition, in case the homogeneous alignment layer treatment is performed in the proper alignment layer treatment direction with the use of the comb-like electrodes shown in FIG. 4, the liquid crystal molecules are twisted in one direction and hence, it gives rise to a phenomenon that the color is changed corresponding to the direction of viewing angle.
FIG. 6 is a schematic view showing the change of color depending on the viewing angle, wherein numeral 30 indicates the bluing direction (direction seen in bluish color) and numeral 31 indicates the yellowing direction (direction seen in yellowish color). Corresponding to the direction to see liquid crystal molecules, the apparent retardation is changed thus giving rise to the bluish color or the yellowish color. The detail of this phenomenon is described in the paper xe2x80x9cJpn. J. Appl. Phys. 36 (1A/B), pp. L27-29 (1997)xe2x80x9d written by S. Aratani et al.
With respect to the above-mentioned two tasks, that is, the asymmetry of the viewing angle characteristics and the color change corresponding to specific viewing angle directions, Japanese Laid-Open Patent Publication 191336/1995, Japanese Laid-Open Patent Publication 105908/1997 and Japanese Laid-Open Patent Publication 258269/1997 disclose measures which can resolve such tasks by changing the shape and arrangement of the comb-like electrodes. In all cases, the viewing angle characteristics become symmetrical in up and down directions as well as in left and right directions and a color change corresponding to the specific viewing angle directions can be suppressed. These methods, however, change the shape of the electrodes and hence, aperture ratio is decreased in most of these measures to resolve tasks. Further, although a method which prevents a decrease of aperture ratio by bending the electrodes has been proposed, this suffers from a problem that color filter substrates which have the bent pixel arrangement become necessary and this pushes up the manufacturing cost.
Accordingly, it is an object of the present invention to j provide a liquid crystal display device which can overcome these problems and can suppress disclination without decreasing aperture ratio.
The liquid crystal display device according to the present invention includes an electrode substrate provided with at least a pair of elongated electrodes, a counter substrate which is arranged such that the counter substrate faces the electrode substrate in an opposed manner and a liquid crystal sandwiched between the electrode substrate and the counter substrate, wherein the liquid crystal display device is constituted such that the device controls the direction of the liquid crystal by an electric field generated between a pair of elongated electrodes so as to perform a display, a pair of such elongated electrodes are arranged such that a long side of one elongated electrode faces a long side of the other elongated electrode in an opposed manner with a given distance, and a conductive member adjusting electric field is disposed between the elongated electrodes such that the conductive member is electrically separated from a pair of elongated electrodes.
Further, according to the present invention, a plurality of one elongated electrodes and a plurality of other elongated electrodes may be arranged so as to constitute comb teeth portions of comb-like electrodes.
Still further, according to the present invention, the rubbing treatment direction of the substrates may be set to be in parallel with long sides of elongated electrode portions and the conductive member may be arranged in a tilted manner by making an angle of not less than 5xc2x0 and not more than 20xc2x0 relative to the long sides of elongated electrode.
Additionally, according to the present invention, the liquid crystal may be made of a twistable liquid crystal.
Still additionally, the conductive member may be made of a metal thin film of approximately up to several 10 nm besides translucent material such as ITO (Indium Tin Oxide) or SnO2. This conductive member is capable of performing its effect even when the conductive member has a width of not more than 3 xcexcm, a thickness of not more than 100 nm and a pretilt angle of not more than 3xc2x0.